1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method.
2. Description of the Related Art
An image forming apparatus comprises a stabilization controller to ensure color stability on a print medium. For example, Japanese Patent Laid-Open No. 1-309082 discloses a technique of reading a toner density detection patch pattern formed on a photosensitive member by a density sensor, and feeding back the read patch pattern to a toner density controller in a developing portion to adjust to the desired density.
In general, a toner patch is easily created and erased, but can provide only density information before fixing. The density information does not reflect the influence after the fixing process and subsequent processes when toner patch-based control is done.
To solve this problem, Japanese Patent Laid-Open No. 62-296669 proposes a technique of reading an image by a reader assembled into the image forming apparatus main body, and controlling image formation on the basis of the read result.
Further, Japanese Patent Laid-Open No. 10-193689 discloses a technique of inserting a sensor in the convey path following fixation in order to sense an output image.
Japanese Patent Laid-Open No. 2002-344759 discloses a technique of adjusting, in correspondence with color detection, the balance (gray balance) of achromatic color to which one is visually sensitive.
It is known that an inkjet printer, which is another example of an image forming apparatus, suffers color variations due to gradual change in the ink discharge amount over time, environmental differences, the individual difference of the ink cartridge, or the like. To prevent this, some products of image forming apparatuses have already been introduced into the market, in which a density sensor is attached beside the ink head to accurately grasp color stability after ink application and control image formation.
Techniques of properly controlling density can be generally classified into control (stabilization control) as disclosed in Japanese Patent Laid-Open No. 1-309082, and control (automatic tone correction) to adjust density tone and the maximum amount of toner application (density) of the color material using the result of printing on a sheet, as disclosed in Japanese Patent Laid-Open No. 62-296669.
A purpose of automatic tone correction is to control the amount of color material by adjusting image forming conditions and provides a stable image forming apparatus by always using a predetermined color material. If the detection signal to detect luminance changes depending on the sheet, the amount of toner application of the color material changes depending on the sheet type. It is, therefore, important to execute calibration using a sheet (reference sheet) recommended by the image forming apparatus manufacturer in automatic tone correction. The recommended sheet generally varies between countries where apparatuses are on sale, and between color and monochrome image forming apparatuses. In most cases, the user does not recognize the sheet type in the sheet feed cassette. Some image forming apparatuses exclusively use output from the manual feeder in automatic tone correction.
From another viewpoint, the reader of the image forming apparatus has a mechanism of correcting, using a reference plate called a shading plate, in-plane no uniformity, gradual deterioration, and contamination. However, it is difficult to arrange a reference plate for a sensor or the like at the fixing convey portion because of space limitations, contamination, and the like. To solve this problem, Japanese Patent Registration No. 03575259 proposes a technique of correcting a sensor using an image-free sheet.
In many cases, the user does not use the recommended sheet in automatic tone correction. In practice, the user executes automatic tone correction with another sheet different from the recommended sheet. If the user does not use the recommended sheet, the amount of toner application of the color material cannot reach a specified amount. The color material is not fixed onto a sheet to cause a fixing offset, and the color material scatters more to degrade the image quality.
Japanese Patent Registration No. 03575259 discloses an arrangement which adopts a reflecting plate in consideration of the sheet transmittance depending on the sheet thickness. However, this arrangement does not consider reflected light at the boundary between the sheet and the color material after transmission. The arrangement in Japanese Patent Registration No. 03575259 cannot cope with sheet type changes (spectral reflectance of the sheet). Also in this case, a mechanism which prompts the user to use the recommended sheet is necessary.
Japanese Patent Laid-Open No. 2004-138710 discloses an arrangement in which the type of print material (plain sheet, OHP, or thick sheet) is detected and a color sensor detects a chromaticity control toner patch fixed on the print material. A plurality of calibration tables specific to print materials are updated on the basis of the detected type of print material and information from the color sensor.
However, Japanese Patent Laid-Open No. 2004-138710 does not disclose an arrangement which executes calibration using a sheet recommended by the apparatus manufacturer in automatic tone correction. Also, Japanese Patent Laid-Open No. 2004-138710 does not disclose an arrangement which determines the type of recommended sheet and whether the sheet is recommended. Japanese Patent Laid-Open No. 2004-138710 does not disclose an arrangement for calibration conditions corresponding to each recommended sheet when adjusting the maximum amount of toner application of the color material in order to implement the maximum amount of toner application of the color material formable by the apparatus. Further, Japanese Patent Laid-Open No. 2004-138710 discloses neither an arrangement which sets the maximum amount of toner application of the color material for a sheet other than the recommended sheet to be smaller than that for the recommended sheet, nor an arrangement which performs luminance-density conversion corresponding to each recommended sheet in tone correction.
For this reason, the arrangement in Japanese Patent Laid-Open No. 2004-138710 can obtain neither a constant maximum amount of toner application of the color material nor proper tone regardless of the type of recommended sheet.